The 2i4 is a great tool for the laptop DJ. Its two phono outputs make connection to a mixer very easy, and the ability to switch the headphone feed source between outputs 1/2 or outputs 3/4 mean that proper cueing is no problem either.
In a typical software DJing program, you will be able to select which outputs are used for your master output and which are used for your cue/monitor output. In this scenario, only one stereo signal needs to be sent to the mixer (and ultimately the PA) as all the cueing is done in the software, by means of the interface itself. An example of this would be outputs 1/2 connected to the mixer for the live feed, and headphones connected to the front of the 2i4, with the headphone source selector set to be 3/4. The feed that is sent to outputs 3/4 (and therefore the headphones) can be controlled in the software by pressing the Cue button, or its equivalent.
Another option for using the 2i4 as a DJing tool more closely resembles that of physical turntables, by means of having outputs 1/2 and outputs 3/4 being the equivalent of each turntable, with both stereo feeds sent to either side of a mixer. In this scenario, the cueing - and crossfading also - would be done on the physical mixer. The 2i4 simply takes the place of the turntables, by providing two different streams of audio.
There are of course other applications for having 4 outputs (often in two stereo pairs) with the headphone source being so easily selectable between the two. The 2i4 can adapt to many different software dependant and personally favoured workflows.
As always, if you have any questions or concerns, you can use the "Contact Us" link above to get in touch.